Presulphuration of hydrocarbon conversion catalysts, primarily for hydrotreatment and hydrodesulphuration and preferably offsite, is carried out in order to fix a sulphur compound on a solid before loading into a catalytic reactor unit. This has been described in our U.S. Pat No. 4,719,195 and European patent EP-A-448 435. During reactor start-up, the sulphur compound decomposes and rapidly and homogeneously sulphurizes the metal oxides present on the catalyst surface to transform them into the sulphide by reaction with the hydrogen present.
The sulphur compound is a polysulphide which is either introduced as a tertiononyl polysulphide by impregnating the catalyst, or a polysulphide is formed on the solid surface by combination of elementary sulphur with an olefinic compound in a vulcanization type reaction (see French patent application 92/04 051).
Reactor start-up generally consists of heating the unit in a hydrogen rich gas. The heating rates are generally between 5 and 50.degree. C./hour.
When the sulphur compound is a polysulphide, sulphur and hydrogen react together in the reactor at temperatures generally between 100 and 200.degree. C., to produce hydrogen sulphide H.sub.2 S which then sulphurizes the oxides in the catalysts. The reactions can be written as follows for a catalyst containing nickel and molybdenum metals: EQU --S--+H.sub.2 .fwdarw.H.sub.2 S (1) EQU MoO.sub.3 +H.sub.2 +2H.sub.2 S.fwdarw.MoS.sub.2 +3H.sub.2 O(2) EQU NiO+H.sub.2 S.fwdarw.NiS+H.sub.2 O (3)
This process operates well and optimally activates the catalysts in the majority of cases. Nevertheless, improvement is necessary in some instances, in particular in two cases which are cited by way of non exhaustive examples. The first concerns catalytic units for the hydrotreatment of crude oil fractions which units are not provided with gas recycling apparatus. In this case, during the activation step in hydrogen with an increasing temperature, a certain amount of H.sub.2 S is formed which is not captured by the metals. This quantity is low, an the order of a few per cent of sulphur with respect to the total weight of sulphur introduced. A first means of limiting this loss is to introduce a slightly higher initial amount of sulphur than stoichiometrically necessary. However, the absence of gas recycling apparatus means that the catalyst is constantly supplied with gas which is free of H.sub.2 S and at high temperature, for example 250.degree. C., 300.degree. C. or 350.degree. C., well above the decomposition temperature of the sulphur compound. The partial pressure of H.sub.2 S in the voids in the catalyst is thus very low. Thus the final stage in the formation of the active phase, namely recrystallization involving atom migration, must be carried out in a non zero partial pressure of H.sub.2 S.
The second instance where conventional techniques are in need of perfecting is where nickel and tungsten-based catalysts are to be activated. Tungsten has a chemistry which is similar to molybdenum, with slight differences: most importantly in this case, the formation kinetics for the active NiS.WS.sub.2 phase are slower than that of the NiS.MoS.sub.2 pair. Thus the final temperature of the activation phase for tungsten is ideally 350.degree. C. to 400.degree. C., whereas for molybdenum based catalysts it is typically between 300.degree. C. and 350.degree. C. At this higher temperature, an absence of H.sub.2 S can seriously and adversely affect phase formation and thus catalyst activity, because of possible formation of WO.sub.2 type species, ie, oxides of quadrivalent tungsten, accompanied by sintering. Nickel and tungsten based catalysts are therefore difficult to activate properly since they are both presulphurized by polysulphide type compounds and the unit is not provided with a gas recycling system.